The basic principle behind electrostatic photocopying processes is the fact that when a paper which is coated with a suitable material such as zinc oxide is subjected to an electric charge it will retain that charge unless exposed to light. By exposing the paper to light the charge is dissipated and this factor can be used to build up an electrostatic image in place of an optical image by forming an optical image of a document or scene onto the charged paper whereupon the charge will be dissipated in areas of the optical image which are brightly lit and will be less dissipated or not dissipated at all in areas which are less brightly lit or are dark. The paper containing the electrostatic charge pattern can then be placed in a solution of carbon black and dispersant or carbon powder can be cascaded over the paper and it is found that the carbon black will be attracted to those areas which are still charged. If employed the dispersant is then allowed to evaporate and the carbon is left on the paper or if a powder has been cascaded onto the paper is impregnated into the paper by being rolled between rollers which may be heated.
By using suitable powders and microencapsulated inks, it has been possible to dispense with the heated rollers and simply cold roll the paper so as to impregnate the latter with the carbon.
Depending on the way in which the optical image is formed, it may be necessary to provide for image reversal within such apparatus and this can either be achieved optically by using additional lenses or an intermediate plate may be used sometimes in the form of a drum on which the electrostatic image is formed and to which the carbon powder or carbon black is attracted and against which an ordinary plain piece of paper can then be pressed so as to transfer the carbon from the plate or drum to the plain paper.